Backport !2934 “gobject: Always ref-sink variants in g_object_set” to glib-2-74
When collecting varargs, ignore the NOCOPY_CONTENTS flag for variants. That is what our docs advice for refcounted types, and it fixes a regression that was inadvertendly introduced when we stopped doing some extra GValue copies.
Includes a test case by Philip Withnall.
Fixes: #2774 (closed)
Trivial backport of !2934 (merged) to glib-2-74
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